1. Field of the Invention
The invention relates to treating by-products of manufacturing processes, in particular, the treatment of the gaseous products generated during the formation of sol-gel bodies.
2. Discussion of the Related Art
Glass optical fiber is typically drawn from a solid preform containing an overcladding that surrounds an inner cladding and core. An overcladding tube is generally formed separately from the inner cladding and core, and the components are then brought together to make the preform. The overcladding does not have to meet purity and uniformity specifications as high as the core and inner cladding, and some efforts at improving manufacturing efficiency and lowering cost of optical fiber manufacturing processes have therefore focused on the overcladding. These efforts have led to the use of sol-gel processes to form overcladding tubes.
U.S. Pat. No. 5,240,488, the disclosure of which is hereby incorporated by reference, discloses a sol-gel process for production of overcladding tubes. In the process, an aqueous colloidal silica dispersion is used. The dispersion is typically stabilized by addition of a base such as tetramethylammonium hydroxide (TMAH). It is also possible to use other tetraalkylammonium hydroxides. TMAH is believed to stabilize silica particles by the following mechanism: Introduction of the TMAH solution into a silica dispersion raises the pH value. The silica then takes on a negative surface charge due to ionization of silanol groups present on the surface. The negative charge on the silica particles creates mutual repulsion, preventing substantial agglomeration and maintaining the stability of the dispersion. At later stages in the process, as discussed at Cols. 14-15 and shown in the Table at Cols. 11 and 12 of the ""488 patent, a polymer (e.g., polyethyloxazoline) is added, a low molecular weight surface modifier (e.g., glycerin) is added, and then a gelling agent (e.g., methyl formate) is added. The gelling agent, through reaction with water and/or base, neutralizes the negatively-charged silica to a degree where gelation is induced.
Subsequent to gelation, bulk water is removed from the sol-gel bodies in a dryer system, and then the bodies placed in a dehydroxylation reactor, within a furnace, for heat treatment. The heat treatment is typically performed in steps which (a) remove remaining interstitial water, (b) remove organic materials, (c) dehydroxylate the sol-gel bodies, and (d) remove metal and oxide impurities. Chlorine-containing gases are often used in such heat treatment both for dehydroxylation and metal impurity removal, as reflected in the ""488 patent.
While processes such as that of the ""488 patent produce good results, the removal of these various impurities from the sol-gel bodies is often problematic. For example, one of the volatile organics generated during heat treatment is trimethyl amine (TRIMA), which exhibits some undesirable properties such as strong odor. The TRIMA therefore generally cannot be released into the environment, but is instead burned in a thermal oxidizer or trapped in a solution and subsequently removed. Other gaseous organic products, as well as the chlorine-containing gases used during the heat treatment, similarly are not able to be released directly into the environment, and liquid residues of organic materials and transition metal impurities must also be disposed in a safe manner.
Apparatus must therefore be designed to trap and treat a variety of gaseous materials resulting from heat treatment of the sol-gel bodies. However, some of the gaseous by-products of the heat treatment tend to condense subsequent to exiting the high temperatures of a dehydroxylation reactor, thereby clogging and damaging the equipment. Other materials, particularly chlorine and thionyl chloride, lead to severe corrosion of metals in the presence of water. Thus, the process and apparatus for treating the numerous gaseous products of a sol-gel heat treatment must be designed to handle a variety of materials, some of which require mutually exclusive treatment conditions. A relatively simple, straightforward design is desired, particular a design suitable for commercial applications.
The invention relates to a process and apparatus for dealing with the variety of gaseous products generated during the formation, specifically the heat-treatment, of bodies such as sol-gel bodies, as well as dealing with the variety of materials that tend to condense on, and clog, conventional equipment. In a process for fabricating sol-gel articles, as illustrated by the apparatus of FIG. 1, gaseous products from a dehydroxylation reactor 10 pass into a gas-fluid contactor 18 through an entrance tube 14, the entrance tube typically heated to at least 150xc2x0 C., more typically at least 325xc2x0 C. (Gas-fluid contactors are known in the art, as discussed in the Chemical Engineers Handbook, Perry and Chilton, ed., 5th Ed., McGraw-Hill, 1973, at 14-2. Such contactors are also known to be used as waste gas scrubbers (see, e.g., Hawley""s Condensed Chemical Dictionary, 12th Ed.)). Heating the entrance tube inhibits condensation of gaseous materials, which would otherwise clog the tube and hinder the process. During the early stages of heat treatment, when TRIMA and other volatile organics are emitted from the dehydroxylation reactor 10, an aqueous acid solution from reservoir 20 is circulated through the contactor 18. The aqueous acid solution traps the basic and/or water soluble organic gaseous materials, and the resultant solution and non-condensing gases pass into the reservoir 20. Specifically, the aqueous acid solution forms a non-volatile salt with the TRIMA, e.g., 
is formed from TRIMA and acetic acid. The reservoir solution, containing aqueous acid as well as organic salts and other dissolved, water-soluble organics, is recirculated through the gas-fluid contactor to continue trapping the gaseous products, with additional aqueous acid solution provided to the reservoir as needed. Volatile, undissolved organics exit with the non-condensing reactor gases and are treated by standard methods, e.g., thermal oxidation or carbon bed adsorption.
Once the organics have been substantially burned out of the sol-gel body, recirculation of the aqueous acid solution is ceased, and the gas-fluid contactor is flushed, typically with water, to wash remaining organic residue from the contactor. The water flow is then ceased and dry nitrogen is flushed through to the contactor. The dry nitrogen enters the contactor through valves 3 and 5, and mixes with the hot gases exiting the reactor. The combined gas stream then flows through valves 1 and 6 into the reservoir and then on to the thermal oxidizer or carbon beds. This gas flow completely dries the interior of the contactor and its associated valvesxe2x80x94otherwise the remaining water, in combination with subsequent chlorine-containing gases from the dehydroxylation reactor, will tend to cause severe corrosion of the equipment. During the next stage of the sol-gel heat treatment, in which such chlorine-containing gases are commonly used, valve 1 is closed, valve 2 is opened, and the gaseous products from the dehydroxylation reactor pass through the contactor into the caustic scrubber. (Certain by-products are condensed within the contactor as solids, e.g., metal chlorides and thermal decomposition products of thionyl chloride.) The dry nitrogen flow is typically continued throughout this stage and for the remainder of the sol-gel body""s heat treatment. The nitrogen flow prevents backstreaming of water vapor from the caustic scrubber and also cools the contactor and the gases exiting the reactor such that condensable metal salts are largely kept in the contactor. Once the heat treatment is complete, valve 2 to the caustic scrubber is closed, valves 1 and 6 are opened to a holding tank 22, and the contactor is flushed, typically with water, to wash the water soluble salts, including metal salts and chlorides, into the holding tank. The whole process is then repeated for a subsequent set of sol-gel bodies.
The invention thus provides a relatively simple and commercially viable way to deal with the variety of gaseous products generated by a sol-gel fabrication process.